As is well known, an oil or gas well can be divided into an upper and a lower part by means of a cement plug which is installed by adding a cement mixture through a supply pipe which ends where the plug is to be located. When the cement mixture is in place, the supply pipe is withdrawn from the cement mixture. The cement mixture is then left alone to harden.
Prior to adding the cement mixture, the well is filled with a liquid, typically a drilling fluid, with a density adjusted to the formation pressure. The cement plug is usually located well above the bottom of the well, and the cement mixture is carried by the drilling fluid until the cement mixture is hardened.
The cement mixture typically has a greater density than the drilling fluid, and the cement mixture therefore tends to sink into the drilling fluid. As a result, a common failure mode for cement plug installations is that the cement plug ends up deeper in the well than planned. Another common failure mode is that the cement plug does not cover the entire cross section of the well resulting in the cement plug coming loose. Occasionally, the cement plug also does not obtain the specified hardness because of its mixing with drilling fluid.
The failure modes often present with cement plug installations are first and foremost due to the fact that the cement mixture has a tendency to sink in one part of the cross section of the well while displaced drilling fluid at the same time rises to the surface through an other part of the cross section of the well. In a vertical well this results in a downward stream of cement mixture and an upward stream of displaced drilling fluids through various parts of the cross section of the well. The result of this is, as previously mentioned, the cement mixture sinks to a much lower level than where the supply pipe ends, and the cement mixture does not cover the whole cross section of the well and the cement mixture is diluted with drilling fluid which reduces its quality.
Thus, in order to secure the formation of a cement plug, it is important to distribute the cement mixture over the whole cross section of the well and to prevent blending with the drilling fluid. An evenly distributed cement mixture prevents the drilling fluid below from escaping to the surface, and a more dense cement mixture floats on top of the drilling fluid below.
One known method of improving cement plug formation is to arrange radial discharge openings in the cement charge pipe in order to distribute the cement mixture evenly around the cross section of the well. It is also a known method to direct the above mentioned discharge openings in an upwards angle to ensure that the cross section of the well is covered with cement mixture prior to a downward movement. Efforts of this type have, however, only limited effects. As mentioned earlier, cement plugs in practice continue to be located deeper than planned, do not cover the entire cross section of the well, or they are too soft. In practice this is typically corrected by building a second cement plug on top of the first one. The result is usually satisfactory, but the cost is unnecessarily high.
It is also known to arrange a well barrier which divides the liquid column in the well into two parts and then supply a cement mixture on top of the well barrier.
A secure well barrier can, with known technology, be achieved by mounting a mechanical plug in the well. It is however time consuming and costly to mount such a mechanical plug, and such known plugs can only be used in the lined part of the well where cross section and other dimensions are known.
One known well barrier which is independent of well diameter and cross section consists of a jelly-like liquid with a density equal to the density of the drilling fluid. The jelly-like liquid will neither sink nor rise, but form a jelly plug. The jelly-like liquid is thixotropic and will harden after a while, but it may take hours before the cement mixture can be poured on top of the jelly plug. A jelly-like liquid which will harden in contact with the cement mixture or its additives is also known.
After the cement mixture is supplied, it is necessary to wait several hours before the location of the plug can be determined and further work be undertaken. By known technology it typically takes 36 hours to complete the setting of a plug as described above.
The present invention is directed to overcoming, or at least reducing the effects of, one or more of the problems set forth above. In particular, the present invention provides a well barrier which can be mounted in a short period of time and which reduces the total time consumed in mounting a cement plug. It further provides a well barrier that ensures the correct location of the cement plug within the well. It still further provides a well barrier that can be mounted in an unlined well and that can be used for large number of well diameters.